1. Field of the Invention
This invention relates to lightweight well cement compositions and methods for forming cement plugs or seals in subterranean zones which can subsequently be removed.
2. Description of the Prior Art
In the drilling and completion of well bores penetrating subterranean formations containing hydrocarbons, voids and other highly permeable zones within the formations are often encountered. Such zones can cause a variety of problems including the loss of expensive drilling or completion fluids thereinto. Heretofore, lost circulation treatments whereby plugging materials such as walnut hulls, mica and cellophane have been used to prevent or lessen the loss of fluids from well bores. The disadvantages of such treatments include the potential for damage to hydrocarbon bearing formations as a result of the inability to remove the plugging materials therefrom, and the dislogement of the plugging materials from highly permeable zones whereby fluid losses subsequently resume.
A more preferable technique for preventing lost circulation problems has been to temporarily plug voids or permeable zones with magnesium oxychloride cement compositions. Solid magnesium oxychloride cements can be removed with minimal damage to hydrocarbon bearing zones or formations by dissolution in acids. However, prior magnesium oxychloride cement compositions utilized in well drilling and completion applications have relatively high densities whereby they can not be utilized in applications calling for low density cements.
Thus, there is a need for low density magnesium oxychloride cement compositions and methods whereby removable plugs or seals can be formed in subterranean zones therewith.